Endoscopic examination techniques have prevailed in a multiplicity of medical and veterinary fields of application, as well as in many non-medical fields. In such examination techniques, an endoscope, which has an elongate shaft with an imaging optical system, is introduced into an internal cavity of a human or animal body or another object to be examined. The elongate shaft that is configured for being inserted into the cavity of the body or other object may be rigid, semi-rigid or flexible. In a distal (i.e. distant from a user) end section of the elongate shaft an endoscope objective is arranged for generating in an image plane an image of an object field in the cavity of the body or object, wherein the endoscope objective typically comprises one or more negative lenses. The generated image is transmitted to a proximal (i.e. close to a user) end of the endoscope for being viewed by the user or picked up by an electronic image sensor which is connected to a display and/or storage device. For transmitting the image from the distal to the proximal end section of the endoscope, the elongate shaft may comprise a fiber optic image guide or a sequence of relay lenses. Alternatively, the image generated by the endoscope objective may be picked up by an electronic image sensor arranged in the distal end section of the shaft and transmitted electronically towards the proximal end section of the endoscope.
Depending on an intended application, endoscopes with various viewing angles are known. In particular, oblique-view endoscopes are known which have a viewing direction that deviates from a longitudinal axis of the shaft. Such oblique-view endoscopes typically have a rigid shaft the distal end of which comprises a cover glass mounted at an oblique angle in a distal opening of the shaft. The cover glass forms a distal window for light rays coming from the object field and entering into the endoscope objective. The objective of such an oblique-view endoscope usually comprises one or several lenses and at least one deflection prism for deflecting light entering at an oblique angle through the distal window into the objective in a direction parallel or almost parallel to the longitudinal axis of the shaft. According to a well-known optical design of objectives for oblique-view endoscopes, the incident light rays, after passing through the cover glass and a first group of objective lenses, enter into the deflection prism through an obliquely arranged entrance face of the deflection prism. On their path through the deflection prism, the rays are reflected twice. The first reflection occurs on a first reflective plane of the deflection prism which is oblique to the longitudinal axis, the rays being reflected back towards the entrance face. The second reflection, which may be a total internal reflection, occurs on the entrance face, which serves as a second reflective plane. After the second reflection, the incident light rays have a direction for entering into another group of objective lenses, being focused in an image plane of the endoscope objective for forming an image of the object field.
According to this optical design, the entrance face of the deflection prism has a twofold function. First, it must permit transmission of the incident rays so that the rays can pass towards the first reflective plane, and second, it must permit reflection, preferably total reflection, of the rays being reflected back by the first reflective plane into a desired direction, which is generally parallel to the longitudinal axis. In order to permit total reflection and to guarantee low losses, the entrance face must provide a sufficient difference of refractive indices from the inside to the outside the deflection prism.
In U.S. Pat. No. 4,815,833, an objective for endoscopes is disclosed comprising an elongated prism on a distal end of which are joined by cementing a diaphragm and a negative lens. A wedge-shaped angular diaphragm, made from metal or opaque glass, is cemented directly to an obliquely cut surface of the distal end portion of the prism. The negative lens is cemented to the distal side of the diaphragm. The image beam from the lens passes through a central opening of the diaphragm and through a surface of the prism that does not have a reflective coating. The beam then strikes another surface of the prism and is reflected back to the first mentioned surface. This time, the angle of incidence is so large that total internal reflection takes place and the image beam is projected along the axis of the prism.
In U.S. Pat. No. 4,850,342, a hard endoscope of oblique-view type is described in which an objective front lens is arranged in parallel with a plane of a cover glass at a slight gap and is fixed to an objective prism with a frame.
Further, it is known to arrange a keyhole-shaped diaphragm between the deflection prism and a negative lens mounted on the deflection prism, in order to provide an air layer on the first surface of the deflection prism that is employed for total reflection.
The positioning of a diaphragm or a frame between the deflection prism and the negative lens is problematic and may increase production cost. Further, dust grains may be caught, reducing the optical quality of the objective.
According to DE 197 36 617 A1, in an endoscope objective having a direction of view including an angle different from zero with the longitudinal axis of the endoscope, on a surface of a prism unit a thin layer is deposited having a refractive index that is smaller than the refractive index of the material of the prism unit. The thin layer provides for total reflection on the corresponding surface of the prism unit. Thus, if an optical element of the objective is placed on the thin layer, no mechanical spacer is needed. However, the thin layer may lead to an increase of production cost, may have non-optimal anti-reflection properties, and the refractive index of the thin layer may undesirably limit the range of incidence angles at which total reflection occurs.
It is therefore an object of the present invention to provide a negative lens and an endoscope objective which alleviate the above mentioned problems. In particular, it is an object of the invention to provide a negative lens for an endoscope objective that can be mounted on a surface of a deflection prism of the endoscope objective in a cost-saving and efficient manner without deteriorating the image quality. It is another object of the present invention to provide an endoscope objective comprising a negative lens that can be assembled in a cost-saving and efficient manner without deterioration of the image quality.
These objects are met by a negative lens and an endoscope objective according to the invention.